Quantitative Chemistry

What is relative atomic mass (Ar)?

Average mass of an atom compared to 1/12th carbon-12

What is relative formula mass (Mr)?

Sum of relative atomic masses of atoms in a formula

How do you calculate Ar?

Weighted average based on isotopes and abundances

How do you calculate Mr for H₂O?

(2 × 1) + 16 = 18

What is a mole?

Amount of substance containing Avogadro's number of particles (6.02 × 10²³)

What is Avogadro’s number?

6.02 × 10²³

What unit is used for moles?

Mol

What is the formula linking moles, mass, and Mr?

Moles = mass (g) ÷ Mr

How would you find the mass from moles and Mr?

Mass = moles × Mr

How do you calculate number of particles?

Number of moles × Avogadro's number

How many atoms in 2 mol of helium?

2 × 6.02 × 10²³ = 1.204 × 10²⁴

What is concentration?

Amount of solute per volume of solution

What is the formula for concentration in g/dm³?

Concentration = mass of solute (g) ÷ volume (dm³)

How do you convert cm³ to dm³?

Divide by 1000

What is the formula for concentration in mol/dm³?

Concentration = moles ÷ volume (dm³)

What is a balanced chemical equation?

Equal numbers of atoms of each element on both sides

Why must chemical equations be balanced?

Conservation of mass

What does the state symbol (s) mean?

Solid

What does (l) mean in a chemical equation?

Liquid

What does (g) mean in a chemical equation?

Gas

What does (aq) mean in a chemical equation?

Aqueous (dissolved in water)

What is percentage mass?

(Mass of element/Mr of compound) × 100%

How do you calculate empirical formula?

Divide atom moles by smallest number, find simplest ratio

What is molecular formula?

Actual number of atoms of each element in a molecule

What is excess reactant?

Reactant not completely used up in a reaction

What is the limiting reactant?

Reactant that runs out first and limits product formed

Why is the limiting reactant important?

It determines the amount of product made

What is atom economy?

(Desired product mass/total reactant mass) × 100%

Why is high atom economy important?

Reduces waste, is more sustainable and cost-effective

What is percentage yield?

(Actual product mass/theoretical mass) × 100%

What are common causes of low percentage yield?

Losses in separation, side reactions, incomplete reactions

Why might a reaction never have 100% yield?

Product loss, reversible reactions, impurities

What is a titration?

Method to find concentration of an acid or alkali

What indicator is used in titration?

Phenolphthalein or methyl orange

What is the end point in a titration?

Point where indicator changes colour (neutralisation)

Why do we repeat titrations?

To get consistent results and calculate an accurate mean

How is a burette used in titration?

Measures known volume of solution added to reaction

What is a standard solution?

Solution of known concentration

Why rinse with distilled water in experiments?

To ensure all reactants/products are measured accurately

What is molar volume of a gas?

One mole of gas occupies 24 dm³ at room temp & pressure

How do you calculate gas volume at RTP?

Volume (dm³) = moles × 24

What does RTP mean?

Room Temperature and Pressure (20°C, 1 atm)

What is the empirical formula of C₂H₆?

CH₃

How do you find empirical formula from masses?

Convert mass to moles, divide by smallest value, write ratio

What is a dilution?

Adding solvent to decrease concentration

What units are used for volume in concentration calculations?

Dm³ (1 dm³ = 1000 cm³)

What is a precipitation reaction?

Reaction where an insoluble solid forms from solutions

What is stoichiometry?

Ratio of amounts of substances in a reaction

What is the sum of all reactant masses compared to product?

Must be equal (conservation of mass)

Why might apparent mass decrease in an unsealed reaction vessel?

Gas escapes

Why is percentage yield rarely 100%?

Loss during transfer, competing reactions, reversible nature

What is a solvent?

Liquid which dissolves the solute to form a solution

How do you prepare a standard solution accurately?

Weigh solute, dissolve in water, transfer to volumetric flask, fill to the mark

Why use a volumetric flask?

Ensures precise, known volume

What is a pipette used for?

Measuring and transferring precise liquid volumes

What does "mean titre" mean?

Average volume of titrant used (excluding rough readings)

Why use concordant titre values?

Ensures accuracy, values within 0.10 cm³

How do you find unknown concentration from titration?

Calculate moles of known; use balanced equation ratio; find unknown using volume

What is a reagent?

Chemical used to cause a reaction or test for something

How do you calculate the number of moles of solute in solution?

Concentration (mol/dm³) × volume (dm³)

Why should you add acid from a burette dropwise near endpoint?

To avoid overshooting endpoint and ensure accuracy

What is back titration?

Indirect method to calculate unknown concentration via excess reagent

What is gravimetric analysis?

Determining substance amount by measuring mass

Give a use of titration in industry.

Checking concentration of chemicals in products

How can a reaction be monitored by gas volume?

Measure gas collected over time in a syringe or measuring cylinder

If 2 moles of H₂O are made from 2 moles of H₂, what is the molar ratio?

1:1

What is a mole ratio?

The proportions of reactants and products in a balanced equation

How do you calculate reacting masses?

Use balanced equation, convert mass to moles, use ratio, convert back to mass

If 40 g NaOH (Mr = 40) reacts, how many moles is this?

1 mole

What is purity?

The proportion of desired substance in a sample

Why is purity important in manufacturing?

Impurities can affect safety and quality of the product

Why might repeated trials differ?

Random error, measurement inaccuracies

What is the effect of increasing volume but keeping moles constant on concentration?

Concentration decreases

What does a high precision mean in measurements?

Readings are close together

How can accuracy be improved in titrations?

Use a white tile, read the burette at eye level, repeat

How do you calibrate a burette?

Fill with known standard solution and check readings

What colour change is observed with phenolphthalein in titration?

Pink (alkali) to colourless (acid)

What safety precaution should you take in titrations?

Wear eye protection, handle acids/alkalis carefully

What is the difference between theoretical and actual yield?

Theoretical is maximum possible; actual is what you get in practice

How do you calculate percentage atom economy?

(Mass of useful products/Total mass of reactants) × 100%

Why is green chemistry important?

Minimises waste and resource use, protects the environment

What is filtrate?

Liquid that passes through the filter paper

What is residue?

Material left on filter paper

What is a systematic error?

Error that affects results in the same way every time

Name a common systematic error in quantitative analysis.

Not zeroing the scale before weighing

What is endpoint in titration?

Exact point where the reaction completes and indicator changes

What is a titre?

The volume of solution added from the burette to reach endpoint

Why plot results on a graph?

To identify patterns, calculate rates, or check for anomalies

What is repeatability?

Ability to obtain consistent results when repeating measurements

How is mean titre calculated?

Average (consistent) titres—exclude rough values

What is meant by significant figures?

Number of digits that reflect measurement accuracy

What is meant by molarity?

Synonym for concentration in mol/dm³

Why swirling is necessary during titration?

To ensure thorough mixing of reactants

What is a meniscus?

Curved surface of a liquid that is read at eye level for accuracy

If you double the solute in solution, what happens to concentration?

It doubles

Explain why burette readings are read from top down.

Graduations decrease as liquid dispenses

How do you check the accuracy of your apparatus?

Use calibration with known values, or compare with reference samples

Summarise why Avogadro’s constant is used.

Links mass, moles, and numbers of atoms or molecules